Microwave signals are transmitted from point-to-point by waveguides or antennas. The main difference between the two is that an antenna radiates the electromagnetic field into open space, while a waveguide confines the electromagnetic field to an area along the signal path. There are a number of conventional waveguide sub-categories.
A transmission line is one such sub-category. It uses some physical configuration of metal and/or dielectrics to direct a signal along the desired path. Typical transmission lines use two conductors—a signal and ground. There are also single conductor transmission lines, such as rectangular waveguides. The simplest type of transmission line configuration is coaxial line. Stripline is essentially a flattened version of the coaxial line configuration. Microstrip line simplifies the stripline configuration, by removing the upper ground planes.
Microstrip line is generally the most commonly used means for planar transmission line applications, because it is highly manufacturable and eases circuit connections and signal probing. Its disadvantage over stripline is that some of the energy transmitted may couple to adjacent traces or into space. Unlike microwave antennas, microwave waveguides are not intended to radiate energy in this way, which causes signal loss and interference. A coplanar waveguide with ground (CPWG) is essentially a low radiation version of microstrip.
In any case, as a microwave signal travels down the signal path of a waveguide, it is subjected to various characteristics associated with that path, such as path discontinuity when transitioning from one path-type (e.g., microstrip line) to another (e.g., component pin). Signal loss is another such characteristic, particularly for long transmission paths. Such characteristics generally impact on the waveguides performance, and must be taken into account.
For example, consider the case where a microwave signal must travel a significant distance on a printed wiring board. This passing of the signal from one point to another is typically achieved with conventional transmission lines, such as stripline, microstrip line, or CPWG transmission lines. The outer conductor or shield of these transmission lines is usually connected to the ground plane.
However, due to their construction, such transmission lines are typically associated with a high and non-uniform current densities in their signal conductors. Generally stated, a non-uniform current distribution across a cross-section of the surface of the center conductor of a transmission line makes for resistive signal conductor losses, and is a dominant loss mechanism. In addition, conventional transmission lines are typically associated with significant manufacturing variability inherent in their assembly process, which further contributes to the poor transition control and discontinuity.
What is needed, therefore, is an improved coax transmission line structure for efficiently carrying and transitioning high frequency signals on a printed wiring board.